The use of alkali metal, ammonium and amine salts of keto-acids as corrosion inhibitors in aqueous systems is known and disclosed, inter alia, in U.S. Pat. No. 4,909,987, U.S. Pat. No. 5,128,396 or EP-A-496 555.
In EP-A-300 325, Example 7, it is taught that the reaction of one equivalent of 3-benzoylpropionic acid with one equivalent of morpholine in ethanol yields a crystalline salt composed, as expected, of one part of acid and one part of base.
It has now been found that the reaction of a keto-acid with a morpholine derivative, without a solvent, or in the presence of an aprotic solvent, yields crystalline complexes composed of two parts of acid and only one pan of base. These crystalline complex compounds are very particularly suitable for use as corrosion inhibitors in coating compositions for protecting metallic surfaces and also for pretreating metallic substrates. The crystalline complex compounds do not lead to a reaction with the paint system, especially an aqueous paint system, and are distinguished by excellent storage stability in coating compositions. They are likewise suitable for both temporary as well as permanent rust protection. These paint systems also exhibit excellent wet bonding strength.